Fix typos in 7.7 and 7.8 PLUS correction of formula in W&W's original

Hi Brian,

When going through 7.7 and 7.8, I found the usual small typos.

In addition, I believe the formula for cotangent expansion is mis-stated in 7.7. In the proof, it was noted that on CD the integrand approaches -il, but this integral is in the negative real direction so, should contribute +il pi. I noticed this when doing the exercises: they don't work out using the formula as stated. 

--Eric